This invention relates generally to calcined clay products, and more specifically relates to a calcined kaolin clay pigment and method of manufacture of same. The pigment produced by the method of the invention has a very low abrasion and very high brightness characteristics when used as a filler or a coating in paper products.
In the course of manufacturing paper and similar products, it is well known to incorporate quantities of inorganic materials into the fibrous web in order to improve the quality of the resulting product. The use of appropriate such fillers vastly improves the opacity and printability of certain types of light weight papers such as newsprint. This aspect of use of calcined kaolin clay pigments is discussed in some detail, for example, in Fanselow and Jacobs, U.S. Pat. No. 3,586,523. Other aspects of the presently preferred commercial methods for manufacturing calcined kaolin pigments for use particularly as fillers in paper manufacture, are also set forth in the said Fanselow et al patent, as well as in additional U.S. patents such as McConnell et al, U.S. Pat. No. 4,381,948.
The Fanselow et al and McConnell et al patents are indeed representative of the methodology widely employed in the kaolin industry in order to produce calcined kaolin clay pigments for use in paper manufacturing. Study of these patents will show that the objective of same, as is customary in the art, is to produce a fine particle size calcined kaolin clay pigment of relatively very high brightness, beginning with a crude kaolin which has a relatively very low brightness. A preferred crude feed material for use in processes such as are disclosed in the Fanselow and McConnell patents, is a highly discolored, so-called "gray" kaolin, which is referred to in the Fanselow patent as a "hard sedimentary kaolin clay." Thus, the gray crude which is used in the example of Fanselow has an initial brightness of 78, where the figure cited refers to the so-called G.E. scale. Procedures for measuring brightness as set forth in this application, and as is generally recognized in the industry, are in accord with TAPPI procedure T646os75. As a result of the beneficiation treatment set forth in the Fanselow et al patent, these brightnesses are considerably increased indeed to a very high whiteness. Claim 2 of the Fanselow et al patent thus recites a G.E. brightness within the range of 92% to 95%. Similarly, the McConnell et al patent describes a resultant pigment having a brightness of at least 93 as being the final output product from practice of the beneficiation methods set forth therein. A calcined kaolin pigment substantially produced in accordance with the McConnell et al patent is available commercially from ECC International Inc. of Atlanta, Ga., under the trademark ALPHATEX.RTM..
It may be noted that both the McConnell et al. and the Fanselow et al. patents are concerned with fully calcined kaolins as opposed to metakaolins. When an uncalcined kaolin is heated (i.e. calcined) to about 1098.degree. F. an endothermic reaction occurs. Essentially all of the water of hydration associated with the uncalcined kaolin crystals is eliminated and an essentially amorphous (as measured by X-ray diffraction) material referred to as "metakaolin" results. If the kaolin is heated to higher temperatures, further significant changes occur. The metakaolin undergoes an exothermic reaction (which typically occurs at about 1700.degree. to 1800.degree. F.). Such material (i.e. which has been heated to at least the exotherm) is then referred to as a "fully calcined kaolin".
In the McConnell patent, it is emphasized that the crude used to produce the high brightness pigments preferably includes not more than 2% by weight of titanium expressed as TiO.sub.2. A principal reason for this is that clay minerals occurring in nature, including kaolin clays, frequently contain their discoloring contaminants in the form of iron and/or titanium-based impurities. The quantities of the titaniferrous impurities in sedimentary kaolins of Georgia are significant and are commonly present as iron oxide-stained titanium oxides. Irrespective of whether calcining is used, it has commonly been considered in the kaolin industry that it is paramount to refine the crude kaolins to bring the brightness characteristics of the resultant product to a level acceptable for various applications such as paper coating, or as mentioned, even for filling. Among the techniques which have been used in the past to remove the discoloring impurities, are the use of hydrosulfites for converting at least part of the iron-based impurities to soluble form, which may then be extracted from the clay. A further method which has come into increasing use in the kaolin industry involves the use of high intensity magnetic separation as described, for example, in such patents as Marston, U.S. Pat. No. 3,627,678. This method is also useful in removing titaniferrous impurities in that although titania when pure has little magnetic attractability, the iron-stained titania which forms the basis (as mentioned) for the bulk of discolorants in many kaolins, may often be quite effectively removed by imposition of such a high intensity magnetic field.
One of the further, very effective methods for removing titaniferrous impurities including iron oxide-stained titanium oxides, is the froth flotation technique. Generally according to this method, an aqueous suspension or slurry of the clay is formed, the pH of the slurry is raised to an alkaline value and a collector agent is added. The slurry is then conditioned by agitating for a short period. A frothing agent if necessary is added to the conditioned slurry, after which air is passed through the slurry in a froth flotation cell to effect separation of the impurities from the mineral.
Further details regarding the use of froth flotation techniques for removing titanium-based impurities from kaolins may be found at numerous places in the prior art, including for example U.S. Pat. Nos. 3,450,257 to E. K. Cundy, 4,518,491 to B. M. Bilimoria, and U.S. Pat. No. 4,090,688 to Alan Nott. In the procedures set forth in these patents, the iron-stained titania contaminants are separated with the froth.
Both the brightness characteristics of the given kaolin and the opacifying properties of same when incorporated as a filler in paper, may be quantitatively related to a property of the filler identified as the "scattering coefficient S". The said parameter, i.e., the scattering coefficient S of a given filler pigment, is a property well-known and extensively utilized in the paper technology art, and has been the subject of numerous technical papers and the like. The early exposition of such measurements was made by Kubelka and Munk, and is reported in Z. Tech Physik 12:539 (1931). Further citations to the applicable measurement techniques and detailed definitions of the said scattering coefficient are set forth at numerous places in the patent and technical literature. Reference may usefully be had in this connection, e.g. to U.S. Pat. Nos. 4,026,726 and 4,028,173. In addition to the citations set forth in these patents, reference may further be had to Pulp and Paper Science Technology, Vol. 2 "Paper", Chapter 3, by H. C. Schwalbe (McGraw-Hill Book Company, New York).
One of the long-recognized concerns that arises where a kaolin clay is subjected to calcination is the increase in abrasiveness, which can result from the formation of various abrasive phases during the calcination process. Such abrasiveness is detrimental to the principal use of the pigments, since among other things, it effects rapid wear at portions of the paper making apparatus. The generation of abrasive phases is a particularly acute problem where the higher temperatures incident to full calcination are employed.
While processes such as are described in the McConnell et al and Fanselow et al patents are primarily based on use of feed kaolins which derive from the fine particle-sized tertiary crudes which are commonly referred to as "hard" kaolins, and while these have become the commercially preferable processes for production of high brightness calcined kaolins for use in paper manufacture, it has also long been known to produce calcined kaolins as well from cretaceous deposits of so-called "soft" kaolins. The general steps involved have included subjecting the crude kaolin to a beneficiation program including such steps as classification, bleaching and the like, and recovery of the beneficiated kaolin for use as feed in a calcining process. In general, however, these prior art techniques have not proved satisfactory for producing a calcined kaolin product which is the equal of or superior to the aforementioned products as produced from the tertiary crudes by the processes described in the patents alluded to. One of the explanations for the superiority of the tertiary-based product may be that the tertiary crudes have an extremely fine particle size, which is known to be a factor in reducing abrasiveness in the final calcined product. Heretofore it has not proved feasible to provide an extremely fine feed from a cretaceous crude in commercially acceptable quantities as would enable a commercially viable calcining process. Much less has such a feed been provided which as well was possessed of the other qualities necessary to produce an outstanding product when calcined, such as an acceptably low titania and iron content.